This invention relates to an improved machine tool precision-measuring apparatus for measuring and rating the machining accuracy of a machine tool.
In a recent field of precision machining technology, a numerically controlled (NC) machine tool is required to provide high-precision machining. The NC machine tool is provided with an actuation-interpolating function for interpolating errors. There are methods for evaluating the function, such as e.g., an actual cutting process and a circular trajectory measuring method.
The actual cutting process includes the steps of: initially cutting a workpiece in practice using an end mill (cutting tool), which is mounted on a spindle of the NC machine tool through a tool holder etc.; then removing the workpiece from the NC machine tool in order to measure using precision measuring equipment how the processed workpiece is shaped; and, assessing errors in such measurements.
The circular trajectory measuring method includes the steps of: mounting a spherical surface receiver on a table of the NC machine tool, which spherical surface receiver has a receiving surface coincident with a sphere, and which receiving surface is made of a magnetically attracting material; positioning the sphere on the spherical surface receiver; mounting a spherical surface seat on the spindle of the NC machine tool, which spherical surface seat is provided with spherical projections, and further which spherical surface seat is made of a magnetically attracting material; placing a length-measuring machine therein, which measures a length using a differential transformer, and then permitting both ends of a radially extending length-measuring bar to be rotatably supported on the sphere and the spherical surface seat with the aid of magnets; then driving the spindle into circular movement about the center of the spherical surface receiver on the table in order to measure variations in length of the length-measuring bar; and, calculating a trajectory of the spindle and then evaluating errors in such a calculation.
However, the actual cutting process is unable to precisely measure a position of the spindle because two different errors commingle in results of the measurement. More specifically, machining errors caused by a cutting tool such as the end mill mingles with errors in trajectories of the NC machine tool.
In addition, the circular trajectory measuring method is unable to accommodate shapes except for an arcuate shape. For example, it is impossible to handle trajectories specified by either linear interpolation or NURBUS interpolation employing a functional equation that expresses a free curve.
In order to overcome the above problems, a measuring apparatus is disclosed in published Japanese Patent Application Laid-Open No. (Hei) 11-58182, as schematically illustrated in FIG. 10.
Referring to FIG. 10, a measuring apparatus 100 is shown having X- and Y-axes directed movable linear guide rails 111, 112 disposed above a base plate 110. These two movable linear guide rails 111, 112 extend in a perpendicular relationship to one another, while being disposed vertically above one another. The X-axis directed movable linear guide rail 111 has both ends positioned on support linear guide blocks 115, 116. The support linear guide blocks 115 and 116 are slid on fixed left and right linear guide rails 113 and 114, respectively. The support linear guide blocks 115 and 116 are movable in the direction of the Y-axis. The fixed left and right linear guide rails 113, 114 are laid on the base plate 110 along opposite edges of the base plate 110 in leftward and rightward directions thereof, respectively. Meanwhile, the Y-axis directed movable linear guide rail 112 has both ends positioned on support linear guide blocks 119, 120. The support linear guide blocks 119 and 120 are slid on fixed front and rear linear guide rails 117 and 118, respectively. The support linear guide blocks 119, 120 are movable in the direction of the X-axis. The fixed front and rear linear guide rails 117, 118 are laid on the base plate 110 along opposite edges of the base plate 110 in forward and rearward directions thereof, respectively. This structure allows the movable linear guide rails 111, 112 to travel in the directions of the Y and X-axes, respectively.
The measuring apparatus 100 has a main block 121 disposed at a position where the movable linear guide rails 111, 112 are perpendicular to one another. The main block 121 is formed by a rectangular box. The main block 121 is movable in the directions of the X- and Y-axes. The main block 121 is connected to a machine tool spindle 125 through a connecting shaft 124. Movement of the spindle 125 causes the main block 121 to be moved in union therewith. In addition, the movable linear guide rails 111, 112 are moved parallel to the respective directions of the Y and X-axes in association with the movement of the main block 121.
The movable linear guide rails 111, 112 have linear scales 126, 127 mounted thereon, respectively. The linear scale 126 is positioned on the top of the movable linear guide rail 111 along substantially the entire length thereof. The linear scale 127 is disposed on the bottom of the movable linear guide rail 112 along substantially the full length thereof. A position-detecting head (not shown) on the main block 121 reads respective graduations of the linear scales 126, 127, thereby allowing positional data on the spindle 125 to be detected. The measuring apparatus 100 having above system is able to measure trajectories of the moving spindle 125 except for circular trajectories of the spindle 125.
However, the prior art measuring apparatus 100 includes a total of six block portions in the directions of the X- and Y-axes, i.e., three for each direction, which block portions are slid on other members upon movement of the spindle 125. In addition, six rail members are required in order to support the block portions. The term xe2x80x9cblock portionxe2x80x9d in this text denotes four-support linear guide blocks 115, 116, 119, 120 and two-through holes of the main block 121. The main block 121 has the through-holes formed therein in the directions of the X- and Y-axes, and further has the linear guide rails 111, 112 inserted through the through-holes. The term xe2x80x9crail memberxe2x80x9d used herein refers to the fixed linear guide rails 113, 114, 117, 118 and the movable linear guide rails 111, 112.
Consequently, the measuring apparatus 100 is complicated in structure and is made heavier in weight because of such a large number of constitutional members.
Furthermore, when the NC machine tools at different locations are to be measured, then the measuring apparatus 100 is so complicated in structure that it takes time to assemble and disassemble the measuring apparatus 100. In addition, the measuring apparatus 100 is heavy in weight, and is thus difficult to move. Further, since the measuring apparatus 100 includes a large number of members, it is time-consuming to adjust the measuring apparatus 100 after assembly thereof.
In view of the above, an object of the present invention is to provide a machine tool precision-measuring apparatus having a simpler structure, lighter weight, and portability.
In the machine tool precision-measuring apparatus fulfilling the above object comprises: a first linear movement distance-measuring means including a first slide shaft, a first slide bush slidably attached to the first slide shaft, and a first distance sensor for measuring a distance that the first slide shaft relatively travels with respect to the first slide bush; and, a second linear movement distance-measuring means including a second slide shaft positioned across the first slide shaft, a second slide bush connected to the first slide bush, the second slide bush being slidably attached to the second slide shaft, and a second distance sensor for measuring a distance that the second slide shaft relatively moves with respect to the second slide bush, wherein a spindle of a machine tool to be measured is fixed to the first linear movement distance-measuring means, while the second linear movement distance-measuring means is fixedly positioned on a base, whereby trajectories of the spindle moving in directions of X- and Y-axes are measured. When the spindle to be measured is fixed to the first distance-measuring means, it is preferred that the first distance-measuring means has a projecting mounting shaft provided on the top thereof, which mounting shaft is secured to a tool holder disposed below the spindle.
In the machine tool precision-measuring apparatus according to the present invention, the spindle of the machine tool is connected to one of the first slide shaft and the first slide bush. The other of the first slide shaft and the first slide bush, which is not fixed to the spindle, and one of the second slide shaft and the second slide bush are connected together in a state of they being positioned across one another. The other of the second slide shaft and the second slide bush, which is not connected to the first linear movement distance-measuring means, is fixedly disposed on the base. This structure provides a reduced number of the slide shafts, or rather two slide shafts, but is able to measure a trajectory of the spindle that provides substantially planar movement.
Thus, the machine tool precision-measuring apparatus has the first and second linear movement distance-measuring means connected together in an intersecting relationship to one another. In such a structure, two slide shafts are enough to measure the trajectory of the spindle moving in a plane. Consequently, the machine tool precision-measuring apparatus has a simplified structure and reduced weight, and is thus convenient to carry.
In the machine tool precision-measuring apparatus according to the present invention, pneumatic bearings using compression gases can be provided between the first slide shaft and the first slide bush and between the second slide shaft and the second slide bush, respectively. Namely, the pneumatic bearings are formed by streams of compressed gases in the gaps between the first and second slide shafts and the first and second bushes, respectively. The first and second slide bushes are complementary to the first and second slide shaft, respectively. The use of the pneumatic bearings permits the first and second slide shafts to be axially slid in non-contact with the first and second slide bushes, respectively. This feature obviates friction-caused exothermicity. In addition, such non-contacting slide provides reduced influence of vibrations, with a consequential increase in positional accuracy.
In the machine tool precision-measuring apparatus according to the present invention, the spindle is preferably joined to the first linear movement distance-measuring means so as to be relatively slidable vertically with respect to the first linear movement distance-measuring means. Alternatively, the first linear movement distance-measuring means is preferably attached to the second linear movement distance-measuring means so as to be relatively slidable vertically relative to the second linear movement distance-measuring means. As a further alternative, the second linear movement distance-measuring means is desirably fitted to the base so as to be relatively slidable vertically in relation to the base. Since the spindle of the machine tool to be measured is moved vertically relative to the base, the preceding structure permits such upward and downward movement of the spindle to be absorbed through sliding portions. As a result, errors in measurements can be reduced as a whole.
In the machine tool precision-measuring apparatus according to the present invention, the first linear movement distance-measuring means is attached to the second linear movement distance-measuring means so as to be relatively slidable vertically in relation to the second linear movement distance-measuring means. In addition, a third distance sensor is preferably provided for measuring a distance that the first distance-measuring means travels upward and downward. Consequently, it is possible to measure a trajectory of the spindle moving in a direction of a Z-axis as well. Such a construction measures the distance of upward and downward movement as well as a horizontal position. As a result, a trajectory of such a spatially moving spindle of the machine tool can be measured.
In the machine tool precision-measuring apparatus according to the present invention, the third distance sensor is preferably disposed on a third linear movement distance-measuring means so as to be able to measure a distance that a third slide shaft relatively travels with respect to a third slide bush. The third linear movement distance-measuring means includes the third slide shaft and the third slide bush. The third slide shaft is slidably attached to the third slide bush.
In addition, the third linear movement distance-measuring means is preferably connected to the first and second linear movement distance-measuring means in such a manner that they are all permitted to measure respective distances in intersecting directions. Further, pneumatic bearings using compressed gases are desirably formed between the first, second, third slide shafts and the first, second, third slide bushes, respectively. The first, second, and third bushes are complementary to the first, second, and third shafts, respectively. Streams of compressed gases are permitted to flow in the gaps between the first, second, third slide shafts and the first, second, third slide bushes, respectively. As a result, errors in measurements can be further reduced.
In particular, when the respective pneumatic bearings are formed between the first, second, third slide bushes and the first, second, third slide shafts, then reduced errors in a vertical direction as well as reduced sliding resistance are attainable for two-dimensional or three-dimensional measurement of the spindle. Such a feature is possible to render measurements more accurate.